text: Pyrophosphoric acid is a tetraprotic acid, with four distinct pK's:
  :, pK = 0.85
  :, pK = 1.96 
  :, pK = 6.60
  :, pK = 9.41 
  The pKas occur in two distinct ranges because deprotonations occur on separate phosphate groups. For comparison with the pKs for phosphoric acid are 2.14, 7.20, and 12.37.
  At physiological pH's, pyrophosphate exists as a mixture of doubly and singly protonated forms.